1. Field of the Invention
This invention relates to a method and apparatus for the manufacture of a continuous sheet of flat glass by supporting molten glass on a pool of molten metal and advancing it along the surface of the pool of molten metal while cooling it to form a continuous sheet of flat glass. More particularly, this invention relates to a method for selectively adjusting and maintaining the condition of the glass at different locations across its width to stabilize the path of movement of the glass as it advances along the surface of the pool of molten metal.
2. Description of the Prior Art
Flat glass may be produced in many different ways. Several methods have been disclosed in the past which involve floating or supporting glass on the surface of a pool of molten metal as it is advanced along that surface and cooled to form a continuous sheet of flat glass. For example, molten glass may be delivered onto a pool of molten metal and formed into a continuous sheet or ribbon of glass according to the teachings of Heal, U.S. Pat. No. 710,357 or of Hitchcock, U.S. Pat. No. 789,911 or according to the methods disclosed in the patents of Pilkington, U.S. Pat. No. 3,083,551 and U.S. Pat. No. 3,220,816 or according to the teachings of Edge and Kunkle found in U.S. Pat. No. 3,843,346. These patents describe processes in which molten glass delivery techniques vary, yet they share the common disclosure that a continuous sheet of flat glass may be formed by advancing a layer of glass along the surface of a pool of molten metal while cooling the glass until it assumes a final width and thickness as a dimensionally stable, continuous sheet of glass.
In the method of Heal, molten glass is delivered over a refractory bridge and then flows slightly downwardly onto the surface of a pool of molten metal confined between two side walls of a forming chamber. This layer of glass advances along the surface of the pool of molten metal between the side walls and remains in contact with them as it is advanced and cooled to form a dimensionally stable, continuous sheet of glass. In the method of Hitchcock, molten glass is delivered through a slot in a refractory wall and flows horizontally onto the surface of a pool of molten metal in a forming chamber. The glass advances along the surface of the pool of molten metal as a layer of constant width and is cooled and advanced at a sufficient speed to form a dimensionally stable, continuous sheet of glass of desired thickness. In the method described by Pilkington, molten glass is delivered through a long, narrow canal and over a refractory lip and then falls freely downwardly onto the surface of a pool of molten metal. It then spreads laterally, outwardly and rearwardly in an unhindered fashion. An advancing layer of glass is drawn along the surface of the pool of molten metal from this laterally, outwardly moving body of molten glass. This layer of glass is advanced as its width diminishes and as it is cooled to form a dimensionally stable, continuous sheet of glass of desired thickness and width. In the method disclosed by Edge and Kunkle, molten glass in a pool of molten glass is conditioned to establish a region of forward flow near its surface, and this forwardly flowing glass is delivered over a threshold member substantially horizontally onto the surface of a pool of molten metal maintained at or near the elevation of the threshold over which the glass is delivered. This flowing molten glass is advanced along the surface of the pool of molten metal as it is initially cooled either having its marginal edges free of contact with side members or having them in contact with selected side members for a short distance. The glass is thereafter advanced along the surface of the pool of molten metal while being further cooled to form a dimensionally stable, continuous sheet of glass. In the method of Heal and the embodiment of the method of Edge and Kunkle wherein the marginal edge portions of the advancing layer of glass in the forming chamber are in contact with side walls or side members, the alignment of the path of the advancing glass is fixed and invariant. However, in the method described by Hitchcock and the embodiment of the method of Edge and Kunkle wherein extended side members are not employed, the alignment of the path of the advancing glass is not fixed by any mechanical means and may be subject to occasional movement or misalignment. In the practice disclosed by Pilkington, alignment of the path of advance of a layer of glass emanating from a spreading body or pool of glass beneath a lip from which molten glass is poured is easily varied since there is neither a physical barrier resisting its movement, nor any directed forward flow within the body of glass which is aligned with the intended path for advancing the layer of glass as it is formed into a continuous sheet of glass. Consequently, in the practice of the method disclosed by Pilkington, instability of the glass position and alignment is persistent. This persistent tendency to become misaligned is characterized as "drifting" or "snaking" of the glass. As indicated, it is a persistent and chronic problem. In the practice disclosed by Hitchcock and in that disclosed by Edge and Kunkle, there is considerably less tendency for the advancing glass to drift from its intended path of advance but some drifting may from time to time occur even in those processes.
Certain solutions to the problem of glass drifting or snaking have been proposed by Barradell-Smith et al. in U.S. Pat. Nos. 3,223,503 and 3,223,509. Methods and apparatus are disclosed in these patents for steering and advancing layer or ribbon of glass on a pool of molten metal in the forming chamber. Disclosed in these patents are a method and apparatus for creating surface movement in the supporting molten metal of a forming chamber so that the movement of molten metal in contact with the undersurface of the glass will act upon the bottom surface of the glass to move the glass. The flow of molten metal within the pool of molten metal may be established using coolers disposed in the molten metal at some location adjacent to the edge of an advancing layer of glass or remote from it to establish thermally induced convection flows. The flow of molten metal may be established using electromagnetic forces from a linear induction motor, such as disclosed by Butler in U.S. Pat. No. 3,453,460. In using the apparatus described in these patents to carry out the methods described in them, the conditions within the supporting molten metal are first altered. These conditions are employed to cause flows in the molten metal beneath the glass supported on it, and these flows then are caused to impose steering forces to the glass itself. Because of this long chain of interactions, the glass responds somewhat sluggishly to changes in the controlled steering devices whether they be linear-induction motors or coolers.
A variety of devices have been disclosed in the art for stabilizing the position of an advancing layer of glass through a forming chamber which involve physically or mechanically engaging a marginal edge portion or both marginal edge portions of the advancing layer, ribbon or sheet of glass, depending upon the location in the forming chamber along the path of advance in which such contact is made. Representative patents disclosing such mechanical steering or ribbon stabilizing devices and methods are as follows: U.S. Pat. No. 3,528,795 to Swillinger shows cooled edge-engaging guide members located on one side of an advancing layer of glass in a forming chamber, particularly in the upstream region of the forming chamber. U.S. Pat. No. 3,506,428 to Itakura shows a device including edge-engaging members for contacting both marginal edges of a continuous sheet of glass that has already been removed from a forming chamber. This device can align a continuous sheet of glass after it is formed but is ineffective to prevent "snaking" or sideward movement of the advancing glass in the forming chamber. U.S. Pat. No. 3,326,653 to de Lajarte et al. shows a method for inserting a flexible, heat-resistant material into each marginal edge portion of an advancing layer of glass, maintaining these in contact with the glass during its cooling and formation as a continuous ribbon of glass and thereafter removing them. U.S. Pat. No. 3,353,943 to Loutte illustrates edge rolls or discs for contacting both marginal edge portions of an advancing layer of glass in the forming chamber and for mechanically maintaining it in a predetermined position.
Functionally, of course, all of the mechanical devices for maintaining the alignment of a continuous sheet or layer of glass advancing along the surface of a pool of molten metal in the forming chamber resist the transverse, lateral movement of the glass in the same general way as do the side walls of the glass forming chamber of Heal. These all provide a positive and direct steering and stabilizing action upon the glass itself, but they typically mark or damage the marginal edge portions of the glass directly due to surface contact or because of the development of optical distortion in the marginal edge portion of the glass due to the drag imposed by the stationary edge-contacting devices acting upon the flowing glass. The steering methods and apparatus of Barradell-Smith et al., on the other hand, do not appear to cause any marking of the marginal edge portions of a continuous sheet of glass, but these methods and apparatus do not provide for rapid and efficient response to variations in glass alignment caused by varying conditions within the glass as it is delivered to the surface of a pool of molten metal.
The present invention provides a method and apparatus for steering and stabilizing an advancing layer or ribbon of glass supported on molten metal in a forming chamber without marking the marginal edge portions of the glass or causing them to develop optical distortion characteristics. The present method and apparatus provide for rapid and efficient response to any intermittent movement of the glass from its intended path of advance.
This invention is practiced in combination with and as an improvement to flat glassmaking processes in which the glass is formed on molten metal. In this process a continuous sheet of flat glass is produced by a method including the following steps: A layer of molten glass is delivered onto the surface of a pool of molten metal maintained within a forming chamber. The molten metal is preferably tin, an alloy of tin or some other metal having a specific gravity greater than the glass and having a melting point lower than the glass to be formed while being substantially nonreactive to the glass at its melting temperature. The layer of molten glass is advanced along the surface of the pool of molten metal and is cooled during such advance to form a dimensionally stable, continuous sheet of glass.
Forces are applied to the glass while it is advanced along the surface of the pool of molten metal. Forces are applied to the glass which are aligned substantially along its path to cause it to be advanced. These forces may be sufficient to cause the glass to be attenuated to a thickness less than an equilibrium thickness during its advance. These forces are characterized as longitudinal tractive forces and may be applied to the glass at any location along the length of the glass sheet. They are preferably applied to the glass at locations well along its path of advance, preferably beyond the supporting pool of molten metal, and are transferred to the hot or more fluid glass primarily due to the surface tension of the glass. Other forces may be applied to the glass in a manner such that they are aligned substantially across or transverse to the path of glass advance. These forces are characterized as transverse forces. The resultant forces caused by the application of the described forces in combination with the reactive surface tension and gravity forces acting on the glass cause the glass to be formed into a continuous, flat sheet of desired thickness.
After the glass has been cooled sufficiently to become dimensionally stable (that is, if it has reached its final width and thickness) it is advanced farther along the surface of molten metal and then is lifted from the surface of the pool of molten metal and conveyed from the forming chamber. It may be lifted slightly and conveyed along a substantially horizontal path from the forming chamber, or it may be lifted and conveyed upwardly from the pool of molten metal in the manner described by Gerald E. Kunkle in his copending, commonly assigned patent application, Ser. No. 483,508, filed June 27, 1974, which is incorporated by reference herein. The glass is cooled sufficiently prior to lifting it from the surface of the pool of molten metal so that its width and thickness remain unchanged during lifting and conveyance from the forming chamber.
As the glass is being formed during its advance along the surface of the pool of molten metal and as it is conveyed from the pool of molten metal, it is controllably cooled. The cooling is coordinated with the rate of glass advance to form a continuous sheet of glass of desired width and thickness. If the layer of glass is permitted initially to spread laterally outwardly in an unhindered manner, the cooling and rate of glass advance are advantageously coordinated to simultaneously attenuate the thickness and the width of the glass in the manner disclosed by Charnock in U.S. Pat. No. 3,352,657 and by Dickinson et al. in U.S. Pat. No. 3,695,859. Nevertheless, a preferred practice involves maintenance of the width of the glass equal to or less than the width of the initially delivered layer of glass for this in and of itself helps to stabilize the path of advance of the glass.
During the advance of the glass along the surface of the pool of molten metal, its path of advance is maintained by practicing this invention.